R. C. Helliwell

Evidence that soil carbon pool determines susceptibility of semi-natural ecosystems to elevated nitrogen leaching

Deposition of reactive nitrogen (N) compounds has the potential to cause severe damage to sensitive soils and waters, but the process of ‘nitrogen saturation’ is difficult to demonstrate or predict. This study compares outputs from a simple carbon–nitrogen model with observations of (1) regional- and catchment-scale relationships between surface water nitrate and dissolved organic carbon (DOC), as an indicator of catchment carbon (C) pool; (2) inter-regional variations in soil C/N ratios; and (3) plot scale soil and leachate response to long-term N additions, for a range of UK moorlands. Results suggest that the simple model applied can effectively reproduce observed patterns, and that organic soil C stores provide a critical control on catchment susceptibility to enhanced N leaching, leading to high spatial variability in the extent and severity of current damage within regions of relatively uniform deposition. Results also support the hypothesis that the N richness of organic soils, expressed as C/N ratio, provides an effective indicator of soil susceptibility to enhanced N leaching. The extent to which current C/N is influenced by N deposition, as opposed to factors such as climate and vegetation type, cannot be unequivocally determined on the basis of spatial data. However, N addition experiments at moorland sites have shown a reduction in organic soil C/N. A full understanding of the mechanisms of N-enrichment of soils and waters is essential to the assessment of current sensitivity to, and prediction of future damage from, globally increasing reactive nitrogen deposition.

Water quality in the Scottish uplands: a hydrological perspective on catchment hydrochemistry

Land above 300 m covers approximately 75% of the surface of Scotland and most of the nations major river systems have their headwaters in this upland environment. The hydrological characteristics of the uplands exert an important influence on the hydrochemistry of both headwater streams and downstream river systems. Thus, many of the spatial and temporal patterns in the chemical quality of surface waters are mediated by hydrological processes that route precipitation through upland catchments. These hydrological pathways also have an important influence on how the hydrochemistry of upland streams is responding to increasing pressures from environmental changes at the global and regional scales. At the present time, atmospheric deposition remains an issue in many parts of the Scottish uplands, where critical loads of acidity are exceeded, particularly in areas affected by increasing N deposition. Moreover, climatic change forecasts predict increasingly wetter, warmer and more seasonal conditions, which may modify the hydrochemical regimes of many river systems, particularly those with a strong snowmelt component. On a more localised scale, land management practices, including felling of commercial forests, expansion of native woodlands, agricultural decline and moorland management all have implications for the freshwater environment. Moreover, increasing public access to upland areas for a range of recreational activities have implications for water quality. Understanding the hydrology of the uplands, through integrated field and modelling studies, particularly of the hydrological pathways that regulate chemical transfers to streamwaters, will remain an important research frontier for the foreseeable future.

Interactive effects of nitrogen deposition and fire on plant and soil chemistry in an alpine heathland.

The response of alpine heathland vegetation and soil chemistry to N additions of 0, 10, 20 and 50 kg N ha(-1) year(-1) in combination with simulated accidental fire (+/-) was monitored over a 5-year period. N addition caused rapid and significant increases in plant tissue N content and N:P and N:K of Calluna vulgaris, suggesting increasing phosphorus and potassium limitation of growth. Soil C:N declined significantly with N addition, indicating N saturation and increasing likelihood of N leakage. Fire further decreased soil C:N and reduced potential for sequestration of additional N. This study shows that alpine heathlands, which occupy the headwaters of many rivers, have limited potential to retain deposited N and may rapidly become N saturated, leaking N into downstream communities and surface waters.

Interaction of nitrogen deposition and land use on soil and water quality in Scotland: issues of spatial variability and scale.

Over large areas of the Scottish uplands anthropogenic sulfur (S) deposition is declining in response to stringent national and European controls on S emissions. At the same time, however, the relative contribution of nitrogenous (N) compounds to the total anthropogenic deposition loading has increased. To investigate the significance of N deposition on the potential acidification of surface waters, national, regional, and catchment databases were developed to assess the relationships between N deposition, soil C/N ratios, land use and surface water NO3 concentrations. National classification schemes for land use and soils were used as only limited empirical data are available at such large spatial scales. Data were screened to eliminate areas where N inputs are dominated by non-atmospheric sources. From these screened datasets, it was apparent that areas with the highest risk of N leaching were situated predominantly in the upland areas of south-west and west Scotland (areas with low soil C/N ratios). At the regional scale, surface-water NO3 concentration in afforested catchments was negatively correlated with soil C/N ratios below 20. This relationship was not evident in moorland catchments, where NO3 leaching was strongly related to N deposition and the loch/catchment ratio, rather than the soil C/N ratio. Temporal trends of regional water quality highlighted as increasing loch NO3 concentrations between 1988 and 1996-1997, presumably reflecting an increase in N deposition, enhanced leaching losses from the terrestrial component of the catchment, or altered in-lake processes. The hydrochemical records for two catchments in NE Scotland (Lochnagar and Allt a Mharcaidh) highlight the importance of within catchment process in controlling the nitrogen response observed in surface waters. The potential mechanisms through which vegetation and soils may modify incoming deposition are discussed.

Effects of future N deposition scenarios on the Galloway region of SW Scotland using a coupled sulphur & nitrogen model (Magic-wand)

The Galloway region of Southwest Scotland has been subject to decades of acidic deposition which has resulted in damage to soils, surface waters and aquatic biota. A survey of lochs was conducted in 1979, 1988, and 1993, over which time there have been dramatic changes in total sulphur and nitrogen deposition. The MAGIC model successfully reproduced the major chemical changes in water chemistry from 1979 to 1988 during which time there was a rapid decline in sulphur deposition. A new coupled sulphur and nitrogen model (MAGIC-WAND) has been used to evaluate the regional hydrochemical response to changing patterns of N & S deposition from the period 1988 to 1993. Details of the model structure and parameterisation are discussed. The model under-predicts the response of non-marine sulphate in the region suggesting that there has been a slight increase in deposition over this period. Future hydrochemical responses to different nitrogen deposition scenarios are presented, indicating that the potential increase of nitrogen in surface waters is closely linked to the age and extent of different mosaics of commercial afforestation within the individual catchments.

Partitioning the variation within the acid neutralizing capacity of surface waters in Scotland in relation to land cover, soil and atmospheric depositional factors

A method of decomposing the variation in the acid neutralizing capacity (ANC) of surface waters in Scotland is described. Using national datasets, a series of variables relating to 703 catchments across Scotland is divided into three components representing (i) land cover, (ii) soil and (iii) atmospheric deposition/altitude. Redundancy analysis (RDA) and (partial) redundancy analysis are used to quantify the amount of variation in ANC uniquely attributable to each of these components, independent of the effects of the others. The variation accounted for by covarying combinations of these components is also determined. Approximately 55% of the total variation in ANC across the 703 sites is explained by the variables representing catchment characteristics and atmospheric deposition. Of this, 8.5%, 2.4% and 6.9% are uniquely attributable to the land cover, soil and deposition/altitude components, respectively. A further 38% of ANC variation is associated with the covariation between components, with 18% accounted for by the combination of all three. Approximately 45% of the variation in ANC remains unexplained. The results reflect the integrated nature of catchment processes and demonstrate, for these data, that it is a combination of land cover, soil and deposition and altitude factors which most explain variation in freshwater ANC level. The approach offers a tool with which to assess the sensitivity of surface waters to acid deposition at a regional scale and provides a way of identifying regional differences in catchment response to acid loading.

Drought alters carbon fluxes in alpine snowbed ecosystems through contrasting impacts on graminoids and forbs

• Climate change is predicted to increase the frequency of drought events in alpine ecosystems with the potential to affect carbon turnover. • We removed intact turfs from a Nardus stricta alpine snowbed community and subjected half of them to two drought events of 8 d duration under controlled conditions. Leachate dissolved organic carbon (DOC) was measured throughout the 6 wk study period, and a (13)CO(2) pulse enabled quantification of fluxes of recent assimilate into shoots, roots and leachate and ecosystem CO(2) exchange. • The amount of DOC in leachate from droughted cores was 62% less than in controls. Drought reduced graminoid biomass, increased forb biomass, had no effect on bryophytes, and led to an overall decrease in total above-ground biomass compared with controls. Net CO(2) exchange, gross photosynthesis and the amount of (13)CO(2) fixed were all significantly less in droughted turfs. These turfs also retained proportionally more (13)C in shoots, allocated less (13)C to roots, and the amount of dissolved organic (13)C recovered in leachate was 57% less than in controls. • Our data show that drought events can have significant impacts on ecosystem carbon fluxes, and that the principal mechanism behind this is probably changes in the relative abundance of forbs and grasses.

Chemical Variation and Catchment Characteristics in High Altitude Lochs in Scotland, U.K.

Previous research has established clear relationshipsbetween the chemical composition of surface waters andthe nature of their contributing catchments. Theserelationships are particularly strong when broadenvironmental gradients are considered. However, forfreshwaters at higher altitudes, some of the catchmentprocesses that mediate chemical composition are lessinfluential than those at lower altitudes. The waterchemistry of 85 upland lochs in Scotland, U.K. is examinedto assess differences in chemical composition along arelatively short altitude gradient. Principal componentsanalysis identifies the main gradients of variationwithin the dataset. A series of digital datasets is usedto characterise the catchments according to a range ofattributes including soils and landcover. Multivariatestatistical analysis is undertaken to examine the extentto which the catchment attributes can explain variationin surface water chemistry in upland systems. Theseempirical relationships may be used in the development ofregionalisation procedures, which will allow upscaling ofknowledge from individual sites to regions.

A Comparison of Loch Chemistry from 1955 and 1999 in the Cairngorms, N.E. Scotland

The Cairngorms in north-east Scotland is remote from pollutant sources although it currently receives ca. 10 kg ha1 yr1 S and ca. 11 kg ha1 yr1 N deposition from the atmosphere.In 1955, 15 lochs (lakes) at a range of altitudes were sampled and analysed for major ion concentrations. A new survey of these and an additional 23 lochs and their catchment soils was conducted in 1999 to determine the impact of acid deposition, and the changes in loch chemistry since the 1955 survey. The bedrock geology of this region has a strong influence on the loch chemistry. Surface waters were generally more acidic in high altitude areas due to predominantly poorly buffered, thin alpine soils developed on granitic parent material (mean acid neutralising capacity (ANC) for 23 lochs = 30 μeq L1). At lower altitudes where the geology is dominated by Dalradian metamorphic rocks surface waters are comparatively base rich and have higher ANC (mean ANC for 15 lochs = 157 μeq L1). Surface water nitrate concentrations show a negative relationship with soil C:N status, in that higher nitrate only occurs at low soil C:N ratios. A comparison of data for 1955 and 1999 shows that sulphate concentrations are significantly lower (67.8 and 47.5 μeq L1, respectively), and pH has improved (pH 5.6 and 5.9) in response to decreased S deposition since the mid 1970s. However, mean nitrate concentrations were found to increase from 2.48 μ>eq L1 in 1955 to 5.65 μeq L1 in 1999. Differences in the sampling and laboratory methods from 1955 and 1999 are acknowledged in the interpretation of data.

Predicting Freshwater Critical Loads from Catchment Characteristics using National Datasets

Maps of freshwater critical loads are used toguide emission strategies for sulphur and nitrogen bothnationally and internationally. Water chemistry data arerequired to calculate critical loads and the production ofnational maps therefore relies on the existence of extensivechemistry datasets. However, the data required to calculatecritical loads are not readily available for all sites. Thisarticle explores how empirical statistical models mightpotentially be used to predict critical loads using nationallyavailable datasets representing a range of catchmentcharacteristics. Initially a global regression model forexplaining freshwater critical load variation across a broadspectrum of catchment types (from lowland agricultural tomountain lakes) throughout mainland Britain is described. Whenattention is focused on more specific catchment types (i.e.upland and non-arable) it is shown that the global model hasless explanatory power. A regionalisation of Great Britain(based on 100 km grid squares) shows that the global modelcannot necessarily be applied successfully within a narrowerregional context. Separate analyses were undertaken on each ofthe regional subsets using backward selection regression. Thevariables emerging as significant predictors variedsubstantially across the regions, as did the explanatory powerof the models. This was also the case when the analysis wasconfined to upland and non-arable catchments. This approachcould be developed so that critical loads assessments can bemade for populations of standing waters rather than simplythose for which water chemistry is available.